The present application is a non-provisional application of International Application No. PCT/FR01/01428, filed May 11, 2001.
The present invention relates to the field of liquid crystal display devices.
Depending on the physical nature of the liquid crystal used, distinctions are drawn between devices in which the liquid crystals are nematic, cholesteric, smectic, ferrolectric, etc. In nematic displays, which constitute the preferred application of the present invention, a nematic liquid crystal is used that is non-chiral or that is made to be chiral, e.g. by adding a chiral dopant. This produces a texture which is spontaneously uniform or lightly twisted, with a helical pitch that is greater than a few micrometers. The orientation and the anchoring of the liquid crystal close to surfaces are defined by alignment layers or treatments that are applied to the substrates.
Most devices that have been proposed or made in the past are monostable. In the absence of an electric field, only one texture is obtained in the device. This texture corresponds to an absolute minimum in the total energy of the cell. Under an electric field, the texture is deformed continuously and its optical properties vary as a function of the applied voltage. When the field is interrupted, the nematic crystal returns to its single monostable texture. The person skilled in the art will recognize that these systems include the modes of operation that are in the most widespread use in nematic displays: twisted nematic (TN), supertwisted nematic (STN), electrically controlled birefringence (ECB), vertically aligned nematics (VAN), etc.
Another class of nematic displays is that of nematics that are bistable, multistable, or metastable. Under such circumstances, at least two distinct textures that are stable or metastable in the absence of a field can be achieved in a cell, using the same anchoring on the surfaces. The terms xe2x80x9cbistablexe2x80x9d or xe2x80x9cmultistablexe2x80x9d are generally used to designate at least two states having the same energy or energy which is very similar, and which are likely to endure almost indefinitely in the absence of any external command. In contrast, the term xe2x80x9cmetastablexe2x80x9d is used for states having different energy levels which are likely to switch after a relaxation time that is long. Switching between two states is achieved by applying appropriate electrical signals. Once a state has been written, it remains stored in the absence of an applied field because the crystal is bistable. This memory of bistable displays is very attractive in numerous applications. Firstly, it enables images to be refreshed at a slow rate, which is most advantageous for reducing energy consumption in portable appliances. Secondly, in fast applications (e.g. video), the memory makes a very high multiplexing rate possible, thus enabling high resolution video to be displayed.
Recently, a new bistable display [document 1] has been proposed, using a liquid crystal that is cholesteric or chiralized nematic. The two bistable textures, U (for uniform or lightly twisted) and T differ from each other by twisting through xc2x1180xc2x0 and they are topologically incompatible (FIG. 1). The spontaneous pitch p0 of the nematic is selected to be close to four times the thickness d of the cell (p0≈4d) so that the U and T energies are substantially equal. Without an applied field, there exists no other state of lower energy: U and T are genuinely bistable.
Because of the topological incompatibility of the two bistable textures, it is not possible to transform one into the other without continuous bulk distortion. Switching between U and T thus requires a strong external field to induce anchoring transitions on the surfaces. Above a threshold electric field Ec (threshold for breaking anchoring), an almost homeotropic texture (referenced H in FIG. 1) is obtained, with anchoring on at least one of the substrates being broken: the molecules extend normally to the plate in the vicinity of said surface.
When the field is interrupted, the nematic molecules close to the broken surface are in unstable equilibrium, without any anchoring torque, and they can return either to their initial orientation (thus returning to the same texture as they had prior to the field being applied), or else they can turn through 180xc2x0 and, after relaxation, give rise to a bulk texture with an additional twist through 180xc2x0. At the end of the command pulse, the cell is guided in selecting one or other of its bistable states depending on whether the coupling between movements of molecules close to the two surfaces is elastic or hydrodynamic: elastic coupling causes a return to the U state, hydrodynamic coupling towards the T state.
For the information displayed on the device to appear, it is necessary for the textures that are achieved to have optical properties that are different. Most devices operate in polarized light and use additional optical components: polarizers, filters, compensating plates, etc. These elements and their orientations relative to anchoring on the two surfaces are selected as a function of the configuration of the display in order to optimize the relevant optical performance: contrast, brightness, color, viewing angle, etc.
For monostable displays, optimization needs to apply to an entire continuum of states achieved under fields of greater or lesser strengths because these states are on display throughout the duration of an image. A very large number of optical configurations have been proposed and made for a variety of devices (TN, STN, etc.), taking account of the particular features of each of those displays.
The optics of a bistable display in which anchoring is broken are very different from those of monostable devices. Firstly, throughout the major fraction of the duration of an image, only two textures are present in each cell of the display: those that correspond to the two bistable states. The optimum configuration must enable maximum contrast to be achieved between those two textures, while minimizing transient optical effects during switching due to passing rapidly through intermediate states under a field. Furthermore, the main difference between the two bistable textures, an additional twist of 180xc2x0, is not a parameter that is available for optimization: it is imposed by the physical mechanism used for achieving two bistable states. In addition, bistable switching requires an electric field that is strong E greater than Ec (close to 10 volts per micrometer (V/xcexcm)) and thus a control voltage U=Exc2x7d that is proportional to the thickness d of the cell. The liquid crystal layer must therefore be very fine (d≈2 xcexcm to 3 xcexcm) in order to make it possible to achieve control with voltages that are reasonable, and so optical optimization must take these requirements into account.
An object of the present invention is now to propose a novel display device based on liquid crystals which present properties that are better than those of previously known devices.
In the context of the present invention, this object is achieved by a reflective bistable device and characterized by the fact that it comprises:
a) a liquid crystal material contained between two parallel substrates, the substrates being provided with electrodes on their facing inside surfaces in order to enable an electric field to be applied to said liquid crystal, at least the front substrate and the front electrode being optically transparent;
b) alignment layers or treatments on the electrodes to orient the liquid crystal and enable at least two alternative distinct textures that are stable or metastable in the absence of a field to be implemented, in which one of the textures is either non-twisted or twisted with a total angle lying in the range xe2x88x9290xc2x0 to +90xc2x0, and the other texture presents additional twisting through an angle close to 180xc2x0;
c) the thickness d of the liquid crystal layer being selected in such a manner that the product dxc2x7xcex94n is close to xcex0/4, where xcex0 is the center wavelength of the working spectrum band of the display and xcex94n is the birefringence of the liquid crystal for said wavelength;
d) means for applying electrical signals to the liquid crystal enabling it to switch between said distinct textures by breaking anchoring on at least one of the two substrates, and enabling the crystal to remain in either texture after the field has been removed;
e) a polarizer associated with the front face of the device, placed inside or outside it, and oriented at an angle lying in the range 15xc2x0 to 75xc2x0 relative to the director of the liquid crystal on the front face of the device; and
f) a specular or diffusing reflective element placed at the rear face of the liquid crystal, inside or outside the device, enabling light to pass twice through the device and return towards an observer or towards additional optical elements.
The reflective bistable display thus proposed by the present invention offers numerous advantages.
In particular it can keep and display an image for a very long time without consuming energy, neither to make it operate (because it is bistable), nor to light it (it does not need an internal light source).
This reflection bistable device can be optimized by taking account of various parameters. With a single polarizer, it makes several configurations possible, giving contrast of 50 to 60 in white light. Without losing optical quality, optimization also makes it possible to use minimum cell thickness, thus making switching much faster and reducing the control voltages needed for switching purposes.
According to other characteristics of the invention:
the liquid crystal material comprises a liquid crystal or a liquid crystal mixture in a nematic phase;
the liquid crystal material comprises a liquid crystal or a liquid crystal mixture in a cholesteric or nematic phase doped with a chiral substance in order to enable the energies of certain textures amongst the stable or metastable textures to be brought closer together or be equalized;
the optical delay dxc2x7xcex94n of the liquid crystal lies in the range 0.15xcex0 to 0.35xcex0, preferably in the range 0.20xcex0 to 0.32xcex0, where xcex0 is the center wavelength of the working spectrum band;
the polarizer is a linear polarizer or an elliptical polarizer;
a compensating plate is introduced on the optical path between the polarizer and the reflector inside or outside the device, providing an optical delay xcex94I less than xcex0/12 where xcex0 is the center wavelength of the working spectrum band;
at least one of the electrodes contains a plurality of different segments enabling a plurality of independent pixels to be made on the same substrate and in the same device;
the independent pixels are provided with independent means for applying the field;
the independent pixels are organized in a multiplexed passive matrix;
the independent pixels are organized in a multiplexed active matrix;
the polarizer is oriented at an angle close to 45xc2x0 relative to the director of the liquid crystal on the front face of the device;
the twist angle of the first texture is practically zero (xcex94xcfx86≈0);
the twist angle of the texture in the low twist state, the relative twist between the two states, the orientation of the polarizer relative to the alignment of the liquid crystal on the front face, the thickness of the liquid crystal material placed between the two substrates, and the birefringence of the liquid crystal are optimized so as to obtain optimum optical performance, in particular in terms of contrast, brightness, and color;
the optical axis of the compensating plate is oriented at substantially 45xc2x0 relative to the polarizer;
the liquid crystal introduces an optical delay lying in the range 100 nanometers (nm) to 180 nm;
the compensating plate introduced an optical delay of less than 50 nm;
the polarizer is combined with the compensating plate in the form of a single element so as to constitute an electrical polarizer; and
the thickness of the liquid crystal material is less than 6 xcexcm.